# Absorption Line's Different Wavelengths

1. Jan 9, 2016

### Atominate

Hey Guys. If an absorption line of hydrogen was observed in the spectrum of the Galaxy, and it's wavelength was 494.9nm, and then the same line was found to be 490nm in the laboratory (in orbit around Earth), what could be drawn from that?
Thanks,
Atominate

2. Jan 9, 2016

### Staff: Mentor

What do you think?
Is this homework?

3. Jan 9, 2016

### Atominate

I am doing a GCSE Astronomy course, but it's not for school. It is a long distance learning course, and this is a question on one of the assignments.

I know that the nm means the nanometers. From the nm of the absorption line, one can draw the temperature and radial velocity. To determine the velocity, one would:
(Observed Wavelength - True Wavelength) divided by the True Wavelength = v/c
(494.9 - 490) dived by 490 = 0.01
0.01 x 300000 (speed of light) = 3000
So we can say, that the Galaxy is blue shifted, and therefore coming towards us at a velocity of 3000 km/s?

4. Jan 9, 2016

### Staff: Mentor

I moved the thread to the homework section.

Does the wavelength get longer or shorter if the galaxy is moving towards us?

You might want to keep one more digit precision in the calculations.

5. Jan 9, 2016

### Atominate

Thanks for moving it to the more specific section. The wavelength would get shorter. What do you mean about the digit?

6. Jan 9, 2016

### Staff: Mentor

But the wavelength you are given here gets longer.

7. Jan 10, 2016

### Atominate

I see, so the wavelength is increasing by 4.9nm meaning the Galaxy is moving away?

I've been told to consider the speed of light as 300,000km/s for this question. Is this what you mean about rounding?

8. Jan 10, 2016

### Staff: Mentor

Right.
No, that is fine. The 0.01 relative shift you calculated is imprecise. There is a huge difference between 0.005 and 0.0149 (a factor of 3!) but both would get rounded to 0.01 if you just keep two digits after the decimal point.

9. Jan 10, 2016

### Atominate

I don't understand what you mean. I am not rounding any of the figures. Can you explain?

10. Jan 10, 2016

### Staff: Mentor

Ah I see, it happens to be "exactly" 0.01. I would still keep one more digit and write it as 0.010.

11. Jan 10, 2016